rg 2 TEXT-BOOK OF PHYSIOLOGY. 



on the existing temperature of the skin, and not upon the absolute 

 temperature of the object.^'Thus, water at 20 C. will evoke a 

 sensation of heat or cold according as the skin has previously been 

 cooled below or warmed above this temperature^ 



The Muscle Sense. As a result of the activities of the muscula- 

 ture of the body or even of its -individual parts, there arises in con- 

 sciousness a series of sensations, which are termed muscle sensations. 

 These sensations give rise to the perception 



1. Of the direction and duration of both passive (due to external 



causes) and active movements (due to internal, volitional efforts) 

 which take place without hindrance; 



2. Of the resistance offered to movements by external bodies; and 



3. Of the posture of the body or of its individual parts. 



As to the seat of the physiologic processes which precede and 

 underlie the development of the sensations two views, at least, may 

 be advanced, viz. : 



1 . That the processes are central in origin and partake of the nature 



of a discharge of nerve impulses from the nerve-cells through 

 the motor nerves to the muscles, the entire process being accom- 

 panied by sensation. This is known as the innervation theory. 



2. That the processes are peripheral in origin, initiated by stimulation 



of specialized end-organs in the muscles and tendons which are 

 connected through the intermediation of afferent nerves with 

 nerve- cells in the cerebral cortex. 



The physiologic mechanism subserving the muscle sense, accord- 

 ing to the second theory, now held by many physiologists, thus in- 

 volves peripheral end-organ's, afferent nerves, their cortical connec- 

 tions and nerve-cells in the cerebral cortex at or near the junction 

 of the superior and inferior parietal convolutions. 



The End-organs. These are small fusiform structures found in 

 and among the muscle bundles of all the muscles of the body with 

 the exception of the diaphragm and eye muscles. In the muscles of 

 the arm and in the small muscles of the hand they are especially 

 abundant. From their shape they are known as muscle spindles. 

 They vary in length from 2 to 12 mm. and in breadth from 0.15 

 to 0.4 mm. Each spindle (Fig. 274) consists of a connective- tissue 

 capsule containing from two to ten longitudinally arranged striated 

 muscle fibers of fme a diameter. In the middle or equatorial region 

 of these intra-jusal fibers there is frequently found a quantity of 

 non- striated protoplasmic matter. The spindle is supplied with both 

 sensor and motor nerves. The sensor fiber loses its external invest- 

 ments as it approaches the capsule. The naked axis-cylinder then 

 penetrates the capsule, and after dividing several times terminates in 

 a ribbon-like or spiral manner around the intra-fusal muscle fiber. 

 This ending was described by and is known as Rufftni's "annulo- 



